This invention relates to an arrangement for operating an electrochemical storage device, particularly a zinc-bromine or zinc chlorine accumulator, in which the electrochemically active material goes into solution during a discharging process and is deposited again on an electrode during a charging process, and in which groups of cells of the accumulator are connected in series to a load. More particularly, this invention relates to such a storage device which is operatively connected to a first auxiliary device for breaking up non-uniformly distributed deposits at the electrodes of the accumulator by completely discharging cells thereof via a resistor and to a second auxiliary device for recharging the cells of the storage device.
The utilization of a zinc-chlorine storage battery in an electric car has been described in Newsweek, June 26, 1980, page 75.
U.S. Pat. No. 4,287,267 discloses a method for bypassing electric current around a defective group of cells of a zinc-chlorine storage system comprising a multiplicity of parallel-connected storage devices. The defective cell group is shorted out of operation by means of a switch and is discharged no further. A power supply for an electrolyte pumping motor, a gas pumping motor and the chlorine gas feed valve are switched off simultaneously with the shorting of the defective cell group. In this manner the operating reliability and availability of a zinc-chlorine storage device can be increased.
Electrochemical storage devices such as zinc-chlorine accumulators are distinguished by high energy density and high power density. In practice, however, the number of charging cycles of such devices are limited. One reason for this limitation is leakage or parasitic currents due to the bipolar arrangement of the electrodes and to the fact that the electrolyte feed takes place via a common main line. The leakage or parasitic currents lead to energy losses and nonuniform zinc deposition (dendrite formation), which effect increases with an increasing number of individual cells combined in a group and is greatest at the electrolyte feeds in the vicinity of the negative end pole of the bipolar cell block. With an increasing number of charge-discharge cycles, normal electrolyte circulation is disturbed and zinc deposition in the individual cells becomes nonuniform, so that finally, the deviations render the storage device inoperative. Furthermore, the formation of dendritic crystals can damage the plates and separators, thereby making the battery useless.
A known method for eliminating parasitic currents consists of opposing to the battery voltage at the electrolyte lines a counter-voltage of about the same magnitude, which counter-voltage compensates the flow of the parasitic currents. The nonuniformity of the crystalline depositions on the electrodes is reduced but not ellminated by this method. Although the total attainable number of charging and discharging cycles prior to inoperability and danger of damage is increased owing to the state of asymmetry in the deposits, this number is still too small for practical application.
A further reason for the generation of nonuniform deposits on the electrodes is seen in the basic mechanisms of the electro-chemical processes, according to which greater deposition continues to occur at already existing crystal deposits owing to increased electric field strength. This necessitates a special treatment of the storage device which consists substantially of a time-consuming complete discharge of the storage device, whereby the deposits are broken up. In the subsequent charging, a uniform new layer is produced again.
During a complete discharge of an electrochemical storage device, the device is not available for energizing a load. In the case of batteries for vehicle propulsion, it is therefore necessary to stock similar storage devices or otherwise a down-time for the entire vehicle occurs.
U.S. Pat. Nos. 3,997,830 and 3,454,859 disclose arrangements for totally discharging a nickel-cadmium accumulator and regenerating and restoring a uniform voltage condition to all cells or groups of cells and for minimizing the degree the efficiency is lowered.
In a design described in U.S. Pat. No. 3,997,830, a solar array and two electrochemical storage devices are connected in parallel to a spacecraft load, the individual cells of the storage devices each being couplable to a discharging resistor. While one storage device is being overhauled and the individual cells thereof are connected to respective discharging resistors by means of a remote control operated from a ground station, the other storage device and the solar battery are available for supplying power to the load. During the discharge operation, the voltage across the terminals of the discharging storage device is monitored and, upon the attainment thereby of a minimum voltage or upon the elapse of a predetermined discharge time, the discharged storage device is charged up again, the charging being monitored by a charge control device.
U.S. Pat. No. 3,454,859 sets forth an arrangement for completely discharging the cells of a nickel-cadmium battery. To increase the life and availability of the battery, the voltage of the entire storage device and of each individual cell is monitored. If the voltage of the entire storage device or of one cell sinks below a predetermined value, the entire storage device is separated from a solar powered charging device and is subsequently discharged via transistor shunted across the storage device. The discharge operation is determined in part by the cell having the lowest voltage. If the voltage of this cell reaches zero, for instance, the discharge of this cell is blocked and the further discharge of the remaining cells is carried out by additional transistors connected in parallel to each cell. Upon the discharge of all the cells, the storage device is reconnected to the solar cell battery for charging.
In the known designs, the load must be connected, during the discharging of the storage device, to another energy source formed by a second storage device or a solar cell system. Such arrangements are of no utility where a second storage device cannot be accommodated for lack of space or for weight reasons.
An object of the present invention is to provide an improved electrochemical power assembly of the above-described type which has an increased availability.
Another object of the present invention is to provide such an electrochemical power assembly for supplying a load in systems in which a second electrochemical storage device cannot be accommodated for weight reasons or for lack of space.
Another object of the present invention is to provide an improved method of operating an electrochemical storage device of the above-described type such that the availability of the device is increased.
A further object of the present invention is to provide a method of operating such an electrochemical storage device for use in systems in which a second back-up storage device cannot be provided for reasons such as weight or space limitations.